Heated debate: politicians air their differences in Taiwan's parliament, the Legislative Yuan, in 2007. Photograph: AP

Though it is often difficult to know what democracy really means, it has never been hard to say where it started, or when: it all began in Athens about 2,500 years ago. Right? Wrong, according to John Keane, who thinks that our taste for these sorts of founding myths is a large part of why we are so confused about what democracy is and how it works. We want the story to be neat as well as noble, and so we lose sight of all the ways in which democracy is neither of these things. Instead, as Keane shows in this monumental new history, it is often chaotic and its progress around the world has been highly contingent, with neither tidy beginnings nor happy endings. Keane's aim is, as he puts it, to "democratise" the history of democracy, opening it up to all the unheralded sources and unlikely experiences that have shaped its fate and ours.

So Athens isn't where it all kicks off: we need to look further back, and further east, to find the origin of the idea that public gatherings could discuss and decide on matters of real importance. The ancient civilisations of Syria-Mesopotamia were experimenting with popular assemblies up to 2,000 years before the Athenians had a go. What marked out Athens was not so much its democratic institutions as its self-importance, its sense of destiny. Keane thinks that we spend too much time thinking about the demos ("people") in Athenian democracy, and not enough worrying about the kratos ("rule").

This wasn't a cosy idea implying order and stability; it "referred to might, strength, triumphant power and victory over others, especially through the application of force". It gave the Athenians a feeling they could make their own fate, competing on equal terms not just with kings, but with the gods as well. They were wrong about this and Athenian democracy was eventually snuffed out by the kings of Macedonia, who included the god-like Alexander the Great. Athens wasn't where democracy was born but it was, as Keane points out, where it suffered its first death.

Still, the idea of democracy didn't disappear along with Athenian pride. One of the things Keane most objects to about our founding democratic myths is the assumption that, as an inherently Western idea, it must have gone into deep storage until rediscovered in England/France/the US (whichever you prefer) about 2,000 years later. In fact, democracy continued to develop in the East, fusing with Islam to produce strange and dynamic hybrids of religious faith, market economics and communal politics. Keane shows that the re-emergence of democracy in the West was not an attempt to break with Eastern traditions, but to compete with them. He locates the earliest European parliaments in 12th-century Spain ("the mother of parliaments", he says, can be found at Léon), where they were a direct response by Christian kings to the threat of democratic Islam, which was threatening to sweep all before it.

Eventually, though, the story has to move west. What Keane calls "assembly democracy" was replaced in the modern era by "representative democracy", with its characteristic mix of mass elections, party political machines and charismatic leaders. Here, the originality of Keane's account lies in his willingness to keep moving south as well: he tells the story of 19th- and early 20th-century Latin American democracy before discussing its more familiar European equivalents. The brutal strongmen and fraudulent caudillo democracies of places such as Mexico and Argentina are held up not as a falling away from some European ideal, but as horrible precursors of the brutality and absurdity of what passed for democracy in "the European graveyard" of the first half of the 20th century.

Keane's preference for "assembly" over "representative" democracy is clear, as is his distaste for what he calls "the pseudo-democratic doctrine of self-determination" that emerged after the first world war and was used to justify "the brazen murdering and herding of peoples". But what he prefers above all is what he thinks has emerged since 1945 - a new kind of democracy he christens "monitory" democracy.

This ugly phrase refers to the multiple and overlapping means citizens now have to scrutinise, complain about and resist their governments, not just through parliaments, but also through watchdogs, audits, regional assemblies, civil society monitors and so on. This isn't as new an idea as Keane implies - the constitutional theorist Benjamin Constant was proposing something similar in the early 19th century - but what is new is the technology now driving it, above all the internet. Keane's central example of monitory democracy in action is India, which is appropriate, given India's distinctive combination of almost limitless political variety bordering on chaos and its increasing technological expertise.

Monitory democracy is an essentially negative idea of politics - it is, as Keane says, the idea of a politics in which "no body should rule". As such, it's only at best a partial description of what democracy is and what it needs to be. In Britain we are living through a monitory democratic moment right now. The expenses scandal shows how much harder it has become for politicians to keep things hidden. But the resulting public rage is also a reflection of how hard we find it to know what democratic politicians are for any more - we can't see what they are doing to justify our having to pay for their swimming-pools. Monitory democracy distances us from politicians by making it increasingly difficult for them to justify any ruling that they have to do. So more and more of the ruling tends to go on behind closed doors. It is no coincidence that Indian politics, for all its dynamism, is just as dynastic as ever.

This is a remarkable book, nearly 1,000 pages long and with something to be learnt from almost every one of them. Still, it's longer than it needs to be. Keane has travelled widely and thought deeply about his subject, but his repeated insistence on the originality of what he's doing starts to grate after a while. He says that his is the first real history of democracy for more than 100 years. It isn't - there was an important (and much shorter) one published by John Dunn just four years ago. What it is is the first history of this type, with this sort of global reach.

Keane seems torn about monitory democracy - he celebrates it, but, in a chapter speculating about the coming century, he foresees all sorts of problems, as people get turned off from politics and violence spills out in ways states can't control. What he doesn't say is that democracy, as well as being a highly contingent idea, has also always been a deeply schizophrenic one. It pulls in different directions. Monitory democracy can function only if it learns to co-exist with some of those democratic ideas that Keane is too quick to dismiss as bogus, from the self-importance of the ancient Greeks to the 20th-century principle of self-determination. Democracy needs something more than the feeling of irritation that comes with being ruled. It needs a sense of purpose - some kind of kratos that the demos can live with.

• David Runciman is the author of Political Hypocrisy: the Mask of Power from Hobbes to Orwell and Beyond (Princeton)

Three to read: ruling passions through the ages

Setting the People Free: The Story of Democracy by John Dunn (Atlantic)

This acclaimed history, published in 2005, charts democracy's course as a concept and form of government from ancient Greece to the 21st century. Dunn argues that although "democracy" became a near universal goal in the 20th century, it is still only the Athenians who have ever tried it in its purest form.

What Price Liberty? by Ben Wilson (Faber)

The story of liberty in Britain, charting its slow emergence from a past of tyranny and repression, through what this young Cambridge historian regards as its heyday in Victorian times, to its uncertain position in the early 21st century.

Democracy: 1,000 Years in Pursuit of British Libertyby Peter Kellner (Mainstream)

This "history" of freedom and democracy in Britain is really a compendium of writings and historical documents, ranging from Magna Carta, Sir Thomas More's Utopia and John Stuart Mill's On Liberty to Barbara Follett's 2005 speech to the House of Commons.Nicole Green